In optical components of a waveguide system such as optical splitters, optical combiners, optical dividers, optical multiplexers and the like, there hitherto are numerous methods of forming optical waveguides on a substrate, which can be categorized generally into two groups that are deposition method and ion exchange method.
Deposition method is the method of forming an SiO2 film on a substrate made of silicon and the like, and it includes the flame hydrolysis deposition method, the plasma assisted chemical vapor deposition method (P-CVD method), the molecular beam epitaxy method (MBE method), and the like, to be more specific.
Referring now to FIG. 3(a)-(c), description is provided hereinafter of a method of manufacturing an optical waveguide using the flame hydrolysis deposition method representing those of the above deposition methods. After fine-grained silicon dioxide (SiO2) added with germanium dioxide (GeO2), or fine-grained silicon dioxide (SiO2) added with titanium oxide (TiO2) is deposited into a thickness of several μm on quartz substrate 7, it is sintered and transparentized to form core layer 8, as shown in FIG. 3(a). Next, the core layer 8 is patterned into a predetermined shape by using the photolithography and dry etching techniques to form core segments 81, as shown in FIG. 3(b). Following the above, silicon dioxide (SiO2) added with phosphorus pentaoxide (P2O5) and boric oxide (B2O3) is deposited over the core segments 81 and the quartz substrate 7 again by the flame hydrolysis deposition method. By sintering, transparent upper cladding layer 9 is formed, as shown in FIG. 3(c), to thus complete the optical waveguide.
On the other hand, described next is a manufacturing method using the ion exchange method. In this method, a borosilicate glass substrate is used, and a waveguide pattern is formed on this substrate by using a metal mask. This substrate is then immersed in molten salt containing dopant, to let the dopant diffuse into the glass within the waveguide pattern by taking advantage of the ion-exchange phenomenon of the dopant with the glass components, and to form a core layer in a surface layer of the glass substrate. Because the ion exchange phenomenon does not occur in an area covered by the metal mask, the core layer is produced only in an exposed area into a shape Identical to that of the mask pattern.
In addition, the glass substrate is immersed in another molten salt containing only components other than the aforesaid dopant among those present In the glass substrate, and an electric field is applied to the substrate to cause the dopant in the core layer formed in the surface layer to migrate toward inside of the glass substrate, to form core segments of a predetermined shape in the glass substrate. These core segments are parts having a high refraction factor so as to become optical waveguides. After removal of the metal mask, the surface layer of the glass substrate is clad with a material of low refraction factor In a manner to cover the core segments, which serve the optical waveguides, into such a configuration that the core segments are buried.
However, there exists the following problems In the deposition method and the Ion exchange method described above.
Although the optical waveguide formed by the deposition method shows the smallest transmission loss, it has a problem in productivity because It requires many processes to make the optical waveguide as It must go through the deposition and sintering processes for the core segments and the cladding layer, and the patterning process using the photolithography method, each of which requires a long duration of processing time. For instance, the core layer requires 1.5 hours for deposition and 10 hours for sintering. On the other hand, although the optical waveguide can be formed In a comparatively short time with the ion exchange method, It produces uneven distribution of the dopant in a direction of thickness of the glass substrate because the core layer is formed by ion exchange. This consequently produces uneven variation of refraction factor in the direction of thickness and increases the transmission loss, thereby giving rise to a problem.